1. Field of the Invention
The present invention relates to a method for making a lithographic printing plate wherein an image-wise exposed precursor is developed with an alkaline developing solution including a compound having at least two onium groups. The present invention relates also to an alkaline developing solution and to a replenishing solution including the compound having at least two onium groups. The present invention relates also to a lithographic printing plate precursor wherein the coating includes the compound having at least two onium groups.
2. Description of the Related Art
Lithographic printing typically involves the use of a so-called printing master such as a printing plate which is mounted on a cylinder of a rotary printing press. The master carries a lithographic image on its surface and a print is obtained by applying ink to the image and then transferring the ink from the master onto a receiver material, which is typically paper. In conventional lithographic printing, ink as well as an aqueous fountain solution (also called dampening liquid) are supplied to the lithographic image which consists of oleophilic (or hydrophobic, i.e., ink-accepting, water-repelling) areas as well as hydrophilic (or oleophobic, i.e., water-accepting, ink-repelling) areas. In so-called driographic printing, the lithographic image consists of ink-accepting and ink-adhesive (ink-repelling) areas and during driographic printing, only ink is supplied to the master.
Printing masters are generally obtained by the image-wise exposure and processing of an imaging material called a plate precursor. A typical positive-working plate precursor includes a hydrophilic support and an oleophilic coating which is not readily soluble in an aqueous alkaline developer in the non-exposed state and becomes soluble in the developer after exposure to radiation. In addition to the well known photosensitive imaging materials which are suitable for UV contact exposure through a film mask (the so-called pre-sensitized plates), heat-sensitive printing plate precursors have also become very popular. Such thermal materials offer the advantage of daylight stability and are especially used in the so-called computer-to-plate method (CtP) wherein the plate precursor is directly exposed, i.e., without the use of a film mask. The material is exposed to heat or to infrared light and the generated heat triggers a (physico-)chemical process, such as ablation, polymerization, insolubilization by cross-linking of a polymer or by particle coagulation of a thermoplastic polymer latex, and solubilization by the destruction of intermolecular interactions or by increasing the penetrability of a development barrier layer.
Although some of these thermal processes enable plate making without wet processing, the most popular thermal plates form an image by a heat-induced solubility difference in an alkaline developer between exposed and non-exposed areas of the coating. The coating typically includes an oleophilic binder, e.g., a phenolic resin, of which the rate of dissolution in the developer is either reduced (negative working) or increased (positive working) by the image-wise exposure. During processing, the solubility differential leads to the removal of the non-image (non-printing) areas of the coating, thereby revealing the hydrophilic support, while the image (printing) areas of the coating remain on the support.
Typically, for a positive-working thermal plate, a dissolution inhibitor is added to a phenolic resin as a binder whereby the rate of dissolution of the coating is reduced. Upon heating, this reduced rate of dissolution of the coating is increased on the exposed areas compared with the non-exposed areas, resulting in a sufficient difference in solubility of the coating after image-wise recording by heat or IR-radiation. Many different dissolution inhibitors are known and disclosed in the literature, such as organic compounds having an aromatic group and a hydrogen bonding site or polymers or surfactants including siloxane or fluoroalkyl units.
The known heat-sensitive printing plate precursors typically include a hydrophilic support and a coating which is alkali-soluble in exposed areas (positive working material) or in non-exposed areas (negative working material) and an IR-absorbing compound. Such a coating typically includes an oleophilic polymer which may be a phenolic resin such as novolac, resol or a polyvinylphenolic resin. The phenolic resin can be chemically modified whereby the phenolic monomeric unit is substituted by a group such as described in WO 99/01795, EP 934 822, EP 1 072 432, U.S. Pat. No. 3,929,488, EP 2 102 443, EP 2 102 444, EP 2 102 445, and EP 2 102 446. The phenolic resin can also been mixed with other polymers such as an acidic polyvinyl acetal as described in WO 2004/020484 or a copolymer including sulfonamide groups as described in U.S. Pat. No. 6,143,464. The use of other polymeric binders in lithographic printing plates are described in WO 2001/09682, EP 933 682, WO 99/63407, WO 2002/53626, EP 1 433 594 and EP 1 439 058.
The positive-working thermal plate may further include, between the heat-sensitive recording layer and the support, an intermediate layer including an alkali soluble resin. This intermediate layer can improve the chemical resistance of the plate against press chemicals. This layer induces an improved removal of the coating on the exposed areas. Typical examples of positive-working thermal plate materials having such a two layer structure are described in, e.g., EP 864420, EP 909657, EP-A 1011970, EP-A 1263590, EP-A 1268660, EP-A 1072432, EP-A 1120246, EP-A 1303399, EP-A 1311394, EP-A 1211065, EP-A 1368413, EP-A 1241003, EP-A 1299238, EP-A 1262318, EP-A 1275498, EP-A 1291172, WO 2003/74287, WO 2004/33206, EP-A 1433594 and EP-A 1439058. However, in the non-exposed areas of these plates, the resistance of the coating for the alkaline developer is poor whereby the difference in the dissolution rate between the exposed and non-exposed areas is insufficient, i.e., the non-exposed areas are partially affected by the developer before the exposed areas are completely dissolved in the developer. As a result, it is difficult to form highly sharp and clear images, particularly highlights, i.e., fine images including a dot pattern or fine lines, are difficult to be reproduced.
In a high quality plate, it is advantageous that such highlights can be reproduced within a sufficient developing latitude, i.e., small fluctuations in developing time do not substantially affect the image formed on the plates and this developing latitude is obtained when the difference in the dissolution rate is improved.
EP 1 182 512 discloses an alkaline developing solution including an amphoteric or cationic surfactant for development of an infrared radiation-presensitized plate.
EP 1 400 856 discloses a method for making a lithographic printing plate whereby the exposed precursor is developed with an alkaline developing solution including a cationic surfactant or a compound having three or more ethylene oxide-terminal groups in the molecule thereof.
EP 1 211 065 discloses a positive-working heat-sensitive printing plate precursor wherein the recording layer includes an organic quaternary ammonium salt.
The printing plates of the prior art suffer from an insufficient developing latitude.